Weeds are plants that are unwanted in a particular environment. For example, in an agronomic environment, weeds are plants that compete with cultivated plants. Weeds can also serve as hosts for crop diseases and insect pests. In agricultural production environments, weeds can cause decreases in crop yield, reduced crop quality, increased irrigation costs, increased harvesting costs, reduced land value, injury to livestock, and crop damage from insects and diseases harbored by the weeds. The principal means by which weeds cause these effects are: 1) competing with crop plants for water, nutrients, sunlight and other essentials for growth and development, 2) production of toxic or irritant chemicals that cause human or animal health problems, 3) production of immense quantities of seed or vegetative reproductive parts or both that contaminate agricultural products and perpetuate the weed species in agricultural lands, and 4) production on agricultural and nonagricultural lands of vast amounts of vegetation requiring disposal. Weeds cost farmers billions of dollars annually in crop losses and weed control expenses.
Chemical herbicides are often used to control the growth and spread of weeds. Chemical herbicides are active at one or more target sites within a plant where they interrupt normal plant functions. For example, the herbicide N-phosphonomethyl glycine, also known as glyphosate, targets EPSPS (5-enolpyruvylshikimate-3-phosphate synthase), the enzyme that catalyzes the conversion of shikimate-3-phosphate into 5-enolpyruvyl-shikimate-3-phosphate, which is an intermediate in the biochemical pathway for creating three essential aromatic amino acids (tyrosine, phenylalanine, and tryptophan).
One limitation on the use of chemical herbicides to control weeds is the emergence of herbicide-resistant weeds. Herbicide resistance is the ability of a plant to survive and reproduce following exposure to a dose of herbicide that would normally be lethal. In weeds, herbicide resistance may occur naturally as the result of random and infrequent mutations. Where chemical herbicide application provides selection pressure, herbicide resistant plants survive to reproduce without competition from herbicide-susceptible plants. This selective pressure can lead to the appearance of increasing numbers of herbicide resistant weeds in a weed population. Herbicide tolerant weeds have been observed for nearly all herbicides in use. There are over 365 weed biotypes currently identified as being herbicide resistant to one or more herbicides by the Herbicide Resistance Action Committee (HRAC), the North American Herbicide Resistance Action Committee (NAHRAC), and the Weed Science Society of America (WSSA). There is a need to effectively manage these herbicide resistant weeds and to provide new compositions and techniques for weed management.